This application is related to application entitled xe2x80x9cCompound Light Source Employing Passive Q-switching and Nonlinear Frequency Conversionxe2x80x9d and application entitled xe2x80x9cPulse Sequencing for Generating a Color Image in Laser-Based Display Systemsxe2x80x9d, both of which are being filed on the same day as this application.
The present invention relates generally to solid state light sources employing lasers with passive Q-switches and nonlinear frequency converters to generate light in the UV wavelength range for writing Bragg gratings and for other photolithographic applications.
Fiber Bragg gratings are quietly revolutionizing modern telecommunication systems and are enabling new types of optical fiber sensors which have the potential to displace equivalent electrical sensor devices. Therefore, it is important to develop suitable apparatus and methods for producing Bragg gratings efficiently and reliably.
Typically, Bragg gratings are written in a photosensitive core of a fiber by illuminating it with an exposure beam at a UV wavelength within a photosensitive range of the core. For example, a Bragg grating is written in a core containing an oxygen deficient matrix in glass (e.g., the core has germanium oxygen deficient centers). Such matrix is highly photosensitive in a range between 240 to 250 nm, where it has an absorption band peaking at about 242 nm. Hence, most commonly employed source of radiation in the UV wavelength range have exposure wavelengths between 240 and 250 nm.
At present, methods for writing Bragg gratings include interferometric techniques, phase mask techniques and point-by-point techniques. There are many variants for each of these three methods, and each requires a suitable light source for generating an exposure beam in the UV wavelength range. Among the most common light source employed for writing Bragg gratings are UV laser sources such as frequency-doubled optical parametric oscillators, narrowed-linewidth 248 nm KrF excimer lasers, intracavity frequency-doubled Argon ion lasers, frequency doubled Ca vapor lasers, frequency quadrupled Nd:YAG lasers. Meanwhile, frequency-doubled optical parametric oscillators pumped by a frequency tripled Q-switched Nd:YAG laser have been used to make fiber Bragg gratings, but such systems tend to be complex and expensive. We note that such systems can be all-solid-state and diode-pumped.
Besides writing Bragg gratings, many materials processing applications include a photolithographic step during which a processed material is exposed to UV radiation. The light sources used for the exposure should be stable, efficient and spectrally pure high-power light sources. For efficient exposure the power level of such light sources should be in the range of several hundred milliWatts and more, e.g., 1 Watt or more. Furthermore, such light sources should be inexpensive to produce and they should generate light in the appropriate portion of the UV wavelength range between 200 nm and 330 nm.
Currently, the most commonly used sources of UV radiation for photolithographic applications such as processing of semiconductor wafers employ excimer lasers of various wavelengths. Excimer lasers at 248 are also the most commonly used UV sources for producing fiber Bragg gratings. Meanwhile, frequency doubled Argon laser emitting at 488 nm (yielding an exposure wavelength of 244 nm) provides the best performance for producing fiber Bragg gratings. Unfortunately, this source is very bulky, cumbersome and expensive to use. For more general information on photolithography using UV radiation the reader is referred to U.S. Pat. No. 5,367,588 to Hill et al. and to U.S. Pat. No. 5,940,568 to Losch et al. addressing the application of photolithographic methods as applied to writing Bragg gratings in fibers.
The prior art teaches various types of light sources for generating light in the visible and UV ranges. A number of these sources rely on a nonlinear frequency conversion operation such as second harmonic generation (SHG) to transform a frequency outside the visible range, e.g., in the IR range, to the desired deep blue or UV frequency. For example, U.S. Pat. No. 5,751,751 to Hargis et al. teaches the use of SHG to produce deep blue light. Specifically, Hargis et al. use a micro-laser which has a rare earth doped microlaser crystal and emits light at about 914 nm to drive SHG in a crystal of BBO producing output at about 457 nm.
U.S. Pat. No. 5,483,546 to Johnson et al. teaches a sensing system for high sensitivity spectroscopic measurements. This system uses a passively Q-switched laser emitting light at a first frequency. The light from the laser is transmitted through a fiber and converted to output light at a second frequency in the UV range. The conversion is performed by two frequency doubling crystals disposed far away from the Q-switched laser.
U.S. Pat. No. 6,185,236 to Eichenholz et al. teaches a self frequency doubled Nd:doped YCOB laser. The laser generates light of about 400 mW power at about 1060 nm and frequency doubles it with the aid of a frequency doubling oxyborate crystal to output light in the green range at about 530 nm. Eichenholz et al. combine the active gain medium and the frequency doubler in one single element to produce a compact and efficient light source.
In U.S. Pat. Nos. 5,745,284 and 5,909,306 Goldberg et al. teach a solid-state spectrally pure pulsed fiber amplifier laser system for generating UV light. This system has a fiber amplifier in a resonant cavity and an acousto-optic or electro-optic modulator incorporated into the cavity for extracting high-peak-power, short-duration pulses from the cavity. These short pulses are then frequency converted in several non-linear frequency conversion crystals (frequency doubling crystals). The addition of the modulator into the cavity for extracting the pulses and placement of the fiber amplifier within the resonant cavity renders this system very stable and capable of delivering a spectrally-pure pulse. Unfortunately, this also makes the system cumbersome and expensive.
U.S. Pat. No. 5,740,190 to Moulton teaches a three-color coherent light system adapted for image display purposes. This system employs a laser source and a frequency doubling crystal to generate green light at 523.5 nm. Moulton""s system also generates blue light at 455 nm and red light at 618 nm by relying on frequency doubling and the nonlinear process of optical parametric oscillation.
Q-switched lasers operating on the 3-level xcx9c980 nm transition of Yb have been demonstrated. For example, in xe2x80x98Three-level Q-switched laser operation of ytterbium-doped Sr5(PO4)3F at 985 nmxe2x80x99 (A. Bayramian, et. al., Opt. Lett. Vol 25, No. 9, Pg. 622-625, May 1, 2000) the authors showed that Yb:SFAP can be Q-switched on this transition, however they had to resort to a complex and inefficient pumping scheme. The authors point out the usefulness of the 2nd and 3rd harmonic of this laser wavelength, but fail to identify the 4th harmonic at 246 nm as attractive. Additionally, they do not indentify writing of fiber Bragg gratings or other photolithographic applications.
Unfortunately, the light sources described above and various other types of light sources taught by the prior art can not be employed to make stable, low-cost, efficient sources of light delivering UV radiation of sufficient power for writing Bragg gratings and other photolithographic applications. This is in part due to the fact that frequency conversion, e.g., frequency doubling in crystals, is not a very efficient operation. If the frequency doubling crystal had extremely high non-linearity, then low power continuous wave (cw) lasers could be efficiently doubled to generate output power levels near 1 Watt. However, in the absence of such frequency doubling crystals high-peak-power, short pulse lasers are necessary to obtain frequency doubled light at appreciable power levels. It should also be noted that providing such high-peak-power short pulses adds complexity to the design of the light sources and introduces additional costs.
U.S. Pat. No. 5,394,413 to Zayhowski addresses the issue of efficient frequency doubling by using a passively Q-switched picosecond microlaser to deliver the pulses of light. Such pulses can be efficiently converted, as further taught by Zayhowski in a frequency-doubling crystal. Devices built according to Zayhowski""s teaching operate at relatively low average power levels and low repetition rates. Attempts to increase these parameters by pumping the microchip harder will cause multiple transverse-mode operation leading to degradation of beam quality and will also incur increased pulse-to-pulse noise.
Hence, what is needed is a stable and efficient source of light in the UV range which can be used for writing Bragg gratings and for other photolithographic applications.
It is therefore a primary object of the present invention to provide a stable, low-cost and efficient light source generating light in the UV wavelength range appropriate for writing Bragg gratings. More specifically, it is an object of the invention to provide such solid state light sources capable of an average power output of several hundred milliWatts, and preferably 1 Watt or more which can be used for writing Bragg gratings in fibers and for other photolithographic applications.
These and other objects and advantages of the invention will become apparent upon further reading of the specification.
The objects and advantages are achieved by a solid state laser source for writing a Bragg grating in a fiber and for other photolithographic applications. The solid state laser source has a mechanism which uses a fiber amplifier for generating a pulsed exposure beam at an exposure wavelength in a UV wavelength range within a photosensitive range of a core of the fiber. The solid state laser source is further equipped with an arrangement for delivering the pulsed exposure beam to the fiber such that the Bragg grating is created in the core. The exposure wavelength is between 240 and 250 nm and preferably between 242 and 245 nm.
The mechanism for generating the pulsed exposure beam preferably has a passively Q-switched laser, the fiber amplifier and at least one frequency conversion element. In one embodiment the frequency conversion is performed by two second harmonic generators set up in series. These two second harmonic generators produce the pulsed exposure beam which corresponds to the fourth harmonic of a pulsed intermediate beam emitted from the passively Q-switched laser. The frequency conversion is performed in a single pass.
The mechanism for generating the pulsed exposure beam preferably has a Yb doped laser emitting at a wavelength between 960 and 990 nm. The Yb doped laser can be a Q-switched laser and preferably a passively Q-switched laser. The Yb doped laser can also be a Q-switched fiber laser. The actual wavelength at which the Yb doped laser emits depends, as is known by those skilled in the art, on the host in which Yb is contained. The Yb doped laser can be a Yb:glass, Yb:YAG, Yb:YLF, Yb:YALO, Yb:FAP, Yb:SFAP, Yb:KY(WO4)2, Yb:ZBLAN. Additional materials which are suitable for use can be found in the open literature and the reader is referred to L. DeLoach et al., xe2x80x9cEvaluation of Absorption and Emission Properties of Yb3+ Doped Crystals for Laser Applicationsxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 29, No. 4, April 1993, pp. 1179-91 for such materials. The preferred materials, due to good overlap between the lasing wavelength and the gain wavelength of Yb-doped germanosilicate optical fiber, are Yb:YLF, Yb:YALO, Yb:FAP, Yb-doped phosphate glass, and other Yb-doped glasses.
In another embodiment the mechanism for generating the pulsed exposure beam has a pulsed diode laser emitting at a wavelength of about 980 nm. The mechanism is further equipped with at least one Yb doped fiber. This fiber can be used for amplifying the output of the pulsed laser diode. In this embodiment the mechanism also has a fourth harmonic generator e.g., in the form of two second harmonic generators set up in series to produce the fourth harmonic of the wavelength emitted by the pulsed diode laser in a single pass.
In yet another embodiment of the invention the solid state laser source has a Yb doped element for emitting the beam at a wavelength of about 980 nm and the fourth harmonic generator for converting that beam to an exposure beam at an exposure wavelength. The exposure wavelength is between 240 and 250 nm, and preferably between 242 and 245 nm. The exposure beam is delivered to the fiber for exposing its core to write the Bragg grating. The Yb doped element can be a pulsed Yb doped element emitting a pulsed beam. In this case, the exposure beam will be a pulsed exposure beam.
The Yb doped element can be a Yb doped laser. Preferably, the Yb doped laser is a Q-switched laser or a Q-switched fiber laser. The Yb doped laser is most preferably a passively Q-switched laser. A fiber amplifier, e.g., in the form of a Yb doped fiber, is used to amplify the output of the Yb doped laser prior to frequency conversion in the fourth harmonic generator.
In one embodiment of a method according to the invention a Bragg grating is written in the core of a fiber. This method calls for providing the solid state laser source with a fiber amplifier, deriving from the source a pulsed exposure beam at the exposure wavelength in the UV wavelength range in a photosensitive range of the core, and exposing the core with the pulsed exposure beam. The exposure can be performed in accordance with an interferometric technique, a phase mask technique or a point-by-point technique. The exposure wavelength is between 240 and 250 nm, and preferably between 242 and 245 nm, since this covers the absorption peak of the core. Specifically, it is preferable that the fiber selected for writing the Bragg grating have an enhanced photosensitivity, e.g., as compared to the SMF 28 fiber standard. The enhanced photosensitivity means that the core has a higher response to the radiation at the exposure wavelength and the Bragg grating can thus be written more rapidly and efficiently.
In another method of the invention a Yb doped element is selected for emitting a beam at a wavelength of about 980 nm. The fourth harmonic at an exposure wavelength is generated by a fourth harmonic generator from this beam. The resulting exposure beam is used for exposing the core. The Yb doped element can be selected to emit a pulsed beam, thereby rendering the exposure beam pulsed.
As will be apparent to a person skilled in the art, the invention admits of a large number of embodiments and versions and can be employed for any photolithographic technique. The below detailed description and drawings serve to further elucidate the invention and its operation.